Engiborg flamethrower/welder fix.

Fixes engiborgs deleting their own welder by creating a flamethrower.
The attackby() of flamethrowers can lead to flamethrower_screwdriver() and flamethrower_rods(). Two new procs that handle the creation of a flamethrower.
New object /obj/item/weapon/weldingtool/largetank/cyborg. This new object does nothing on the flamethrower creation procs.

I'm not entirely happy about this.